A specific 55,000 MW phosphoprotein was found in murine embryo cells. This protein is very similar, if not identical, to an SV40 induced protein, isolated earlier from SV40 transformed cultured fibroblasts. The amount of the embryo phosphoprotein is high in cells taken from midgestation stage mouse embryos. We are studying the biochemical nature and the possible function(s) of this embryonal protein. This (or similar) protein(s) appear in stable form in cultured cells which were transformed by various other means including DNA and RNA viruses, but in much lower amounts in normal or spontaneously transformed cells. The amount of the stable protein is not a correlate of cellular tumorigenicity or cell division rate. The function of this embryo protein is unknown.